Improve gradle plugin tags documentation

See gh-33614

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/asciidoc/packaging-oci-image.adoc

@@ -179,7 +179,7 @@ The value supplied will be passed unvalidated to Docker when creating the builde
 
 | `tags`
 |
-| A list of one or more additional tags to apply to the generated image.
+| A list of one or more additional tags to apply to the generated image.The values provided to the `tags` option should be full image references in the form of `[image name]:[tag]` or `[repository]/[image name]:[tag]`.
 |
 
 | `buildCache`